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As the Cedar River rose higher and higher, and as he stacked sandbags along the levee protecting downtown Cedar Falls, Kamyar Enshayan, a college professor and City Council member, kept asking himself the same question: "What is going on?"

The river would eventually rise six feet higher than any flood on record. Farther downstream, in Cedar Rapids, the river would break the record by more than 11 feet.

Enshayan, director of an environmental center at the University of Northern Iowa, suspects that this natural disaster wasn't really all that natural. He points out that the heavy rains fell on a landscape radically reengineered by humans. Plowed fields have replaced tallgrass prairies. Fields have been meticulously drained with underground pipes. Streams and creeks have been straightened. Most of the wetlands are gone. Flood plains have been filled and developed.

"We've done numerous things to the landscape that took away these water-absorbing functions," he said. "Agriculture must respect the limits of nature."

Officials are still trying to understand all the factors that contributed to Iowa's flooding, and not everyone has the same suspicions as Enshayan. For them, the cause was obvious: It rained buckets and buckets for days on end. They say the changes in land use were lesser factors in what was really just a case of meteorological bad luck.

But some Iowans who study the environment suspect that changes in the land, both recently and over the past century or so, have made Iowa's terrain not only highly profitable but also highly vulnerable to flooding. They know it's a hard case to prove, but they hope to get Iowans thinking about how to reduce the chances of a repeat calamity.

"I sense that the flooding is not the result of a 500-year event," said Jerry DeWitt, director of the Leopold Center for Sustainable Agriculture at Iowa State University. "We're farming closer to creeks, farming closer to rivers. Without adequate buffer strips, the water moves rapidly from the field directly to the surface water."

Corn alone will cover more than a third of the state's land surface this year. The ethanol boom that began two years ago encouraged still more cultivation.

Between 2007 and 2008, farmers took 106,000 acres of Iowa land out of the Conservation Reserve Program, which pays farmers to keep farmland uncultivated, according to Lyle Asell, a special assistant for agriculture and environment with the state's Department of Natural Resources (DNR). That land, if left untouched, probably would have been covered with perennial grasses with deep roots that help absorb water.

The basic hydrology of Iowa has been changed since the coming of the plow. By the early 20th century, farmers had installed drainage pipes under the surface to lower the water table and keep water from pooling in what otherwise could be valuable farmland. More of this drainage "tiling" has been added in recent years. The direct effect is that water moves quickly from the farmland to the streams and rivers.

"We've lost 90 percent of our wetlands," said Mary Skopec, who monitors water quality for the Iowa DNR.

Crop rotation may also play a subtle role in the flooding. Farmers who may have once grown a number of crops are now likely to stick to just corn and soybeans -- annual plants that don't put down deep roots.